8 



12 



2 4 6 



Total salts (ppt) 



Figure 4-3. Comparison of soil calcium and total salt 

 concentrations within four marsh types in 

 coastal Louisiana (Palmisano and Cha- 

 breck 1972). 



A second important aspect of a habitat is its areal 

 extent. The habitat's area relates to the relative 

 amount of space, food and cover available for a par- 

 ticular species. Some species, e.g., the river otter, 

 occur in low densities and range over large areas. For 

 these species, a large continuous area of habitat is 

 necessary to maintain the population and its gene pool. 



A third characteristic of a habitat that is impor- 

 tant to individual populations is its interlinking with 

 other habitats. Habitats are interlinked by populations 

 that migrate from one to another. For instance, 

 virtually all of the commercial and sportfish species in 

 the Chenier Plain must use several habitats (near- 

 shore Gulf, inland open water, and one or more wet- 

 land habitats) to complete their life cycles. 



4.1.2 RELATION OF HABITATS TO BASINS 



In part 3.0 habitats were discussed as components 

 of basins and were considered as geographic units of 

 defmed area. Habitats receive sunlight, water, sedi- 

 ments and nutrients. With coupling among habitats 

 they can be expected to produce a characteristic 

 harvest of commercially important fishes, shellfishes 

 and mammals, and sustain a given level of recrea- 

 tional use. Water unifies the basin, acting as a vehicle 

 for transport of materials and organisms among habi- 

 tats. Renewable resource potential was discussed as a 

 function of areal habitat change, not as a result of 

 changes in habitat quality. 



In part 4.0 the emphasis changes to the biotic 

 components of each habitat; the complex trophic 

 structure, the major processes, and reactions of the 

 habitat to external forces are presented. The dis- 

 cussion is intended to show how processes which lead 

 to changes in habitat area and quality at the basin 

 level can also lead to changes in habitat structure, 

 function, and carrying capacity. 



4.1.3 ORGANIZATION OF HABIT.\T SECTION 



Wetlands appear to be the most important habi- 

 tats in the Chenier Plain in terms of loss and vulnera- 

 bility to change. They are therefore treated first, fol- 

 lowed by aquatic habitats, beach and ridge habitats, 

 upland forest habitat, and finally agricultural habitats. 

 Within each of these groups, functional similarities far 

 outweigh differences. Therefore, a general discussion 

 of common characteristics within each group prefaces 

 individual habitat treatments. Differences, especially 

 in species composition, are considered in subsections 

 on individual habitats. 



4.2 WETLAND HABITATS 



From the air wetlands appear as watery grasslands 

 interspersed with countless lakes, ponds, and sinuous 

 streams, along whose borders the vegetation is par- 

 ticularly lush. From low altitudes, different types of 

 vegetation can be distinguished in broad bands, lying 

 rouglily parallel to the major water bodies and to the 

 coast. 



The five distinct natural wetland habitats are 

 identified by their dominant vegetation (Penfound and 

 Hathaway 1938; O'Neil 1949; and Chabreck 1970, 

 1972). These habitats function as they do, and occupy 

 particular spatial relationships to each other, pri- 

 marily because of tlie influence of two related hy- 

 drologic processes. Freshwater from rainfall and from 

 up-river discharge flows seaward across the basin. 

 Saltwater influenced by tidal currents and density 

 gradients from the Gulf tends to oppose this flow. 

 Depending on the topographic features of the basin, 

 the mixing of fresh and saltwater results in different 

 marsh zones (fig. 4-4). Each of these zones supports 

 a characteristic fiora. 



Three additional points should be made. First, 

 wetland habitat boundaries are often not distinct. 

 Salt concentrations vary over a continuum, not 

 abniptly, and vegetation zones also grade diffusely 

 into each other and overlap much as an ecotone 

 separates a field from a forest. Second, wetland 

 habitats usually occur as an ordered series. Fresh 

 marsh is not expected to be contiguous to salt marsh, 

 rather, there is a transition through intennediate and 

 brackish zones. This follows from the mixing processes 

 that produce a gradual change in salinity across the 

 basin. Finally, the habitat zones change dynamically 

 through time in response to natural and culturally 

 induced changes in the hydrologic regime. For ex- 

 ample, as natural subsidence occurs, seawater en- 

 croaches over the land, and marsh habitat boundaries 



156 



